What's the best PC/Desktop?
A PC or Desktop which has been assembled, is considered best as you can choose the best compatible parts for the build and it has a lot of options for hardware customization and hardware upgrades. On the other hand, selecting a branded machine gives very less options for the compatible parts and has limited settings in BIOS.

Motherboard : There are several motherboards from different manufactures but GIGABYTE motherboards always stands at the top. The reason is they are best selling motherboard and they've maximum compatibility for Hackintosh. Has everything easy to access, setup, configure and they're ultra durable. Beside this, it has a number of advantages such as Dual UEFI BIOS. The next motherboard which stands is ASUS. The rest are good and are compatible for sure. When buying a motherboard, you should look for the other details described below.

CPU : The important thing to start off is having a good CPU which is compatible with macOS! It depends on your work need but still, minimum Core i3, Arrandale. But, how can I identify my CPU specifically? Well, a Sandy Bridge CPU has a name like iX-2xxx*, Ivy Bridge iX-3xxx*, Haswell iX-4xxx*, Broadwell iX-5xxx*, Skylake iX-6xxx*, Kaby Lake iX-7xxx* and Coffee Lake iX-8xxx*.

Note :

CPUs without the support of the SSE4 instruction set won't work with Sierra and later.

Graphics : It's good to have a external GPU such as nVidia or AMD. AMD has several advantages over nVidia and works even updating your macOS. Also, if there's new release of a macOS, the AMD cards would be compatible. With the release of High Sierra, several RX cards are supported out of the box with few minimal edits. If you're having a tight budget or want to use Intel HD Graphics(integrated graphics), you can also use it. However it requires few edits and tuning.

To add some more info about the GPUs, if you are looking for a better compatibility on the GPU part it's highly recommended to use AMD GPUs over the nVidia GPUs. Although not all AMD GPUs are Macintosh or Hackintosh compatible but almost all the RX (400, 500, and Vega) Series are compatible with the exception of RX550 (you can make them work but it needs some extra steps).

Following are the list of supported Intel HD Graphics.

RAM: No doubt, the RAM are compatible for sure. You can choose any brand, any frequency. But make sure your motherboard supports the RAM type and frequency. DDR1, DDR2, DDR3, and DDR4 are the memory types.

Ethernet : For ethernet, it's built-in. The ethernet chips are from different manufacturers such as Intel, Atheros, Realtek, Broadcom. Now a days, Intel ethernet chips are becoming more popular and have fast connection speeds. Some motherboard which offers a Dual LAN, can create issues such as panics or crashes. Better if you avoid those.

Audio : Most of the motherboard comes with a Realtek Chip, on board. There are some other manufacturers like VIA VT, these days, the Realtek ALC1220 is the popular codec and has a good compatibility and the quality is great. Realtek Audio Codec has less trouble when enabling the audio on hardware. Other codecs such as VIA VT, Creative can cause issues and may not work properly.

Wi-Fi/Bluetooth : Most of the board doesn't have Wi-Fi/Bluetooth with them, except few likely when having a ITX Build. But having a WiFi and bluetooth is a good feature and removes the requirement of using a long cable running from your router to your PC. You can either use a PCIe based WiFi Card or can also use a external WiFi Adapter. The PCIe based cards are fast and can be installed in the motherboard without occupying your USB ports. Few cards which are compatible works out of the box. These cards are often server the purpose of WiFi but few PCIe based cards, can have both WiFi and Bluetooth as well so we recommend to use those cards. Using USB WiFi and Bluetooth Adapters can cause issues like sleep or can even have panics. So try avoiding as much as you can. For more information, see the notes below.

It's better to use WiFi Cards with the Broadcom chipsets because so far with macOS Mojave DP11 there's no more support for WiFi cards with Atheros chipsets.

There is a workaround which requires extracting the AirPortAtheros40.kext inside the the IO80211Family.kext/Contents/Plugins folder from high Sierra 10.13.x and installing it inside /System/Library/Extension on macOS Mojave.

The Bluetooth from the onboard(Intel) is supported but not fully. Some features such as handoff/hotspot/unlocking with Apple watch may not work.

Hard Drives/SSD/M.2: All the hard drives and SSDs with SATA interface are compatible. With the release of High Sierra, the M.2 drives are fully supported. You can choose any brand. Samsung is the world leading in terms of storage drives and offers a very fast speed when compared with other brands. So the Samsung is the recommended one. M.2 SSDs comes in different form factors. make sure your motherboard provides a compatibility for the form factor you choose.

Power Supply: Selecting a power supply is very crucial. Select the supply according to the peripherals you have. A recommend power supply for a ITX build having a external GPU, K CPU is 500W-600W supply and increasing with the board form factor and the devices installed. Also make sure there are sufficient connectors for your peripherals and the motherboard.

Keyboard/Mouse: All Keyboard/Mouse are compatible. Either PS/2 or USB interface.

And you know about the cases and customization.
Now, it's your turn to do the things!
Good luck!